Pt-Pd diesel oxidation catalyst with CO/HC light-off and HC storage function

ABSTRACT

The present invention is directed to a diesel oxidation catalyst for the treatment of exhaust gas emissions, such as the oxidation of unburned hydrocarbons (HC), and carbon monoxide (CO) and the reduction of nitrogen oxides (NOx). More particularly, the present invention is directed to a novel washcoat composition comprising two distinct washcoat layers containing two distinctly different ratios of Pt:Pd.

FIELD OF THE INVENTION

The present invention is directed to a Pt/Pd diesel oxidation catalystwith CO/HC light-off and HC storage functions. More specifically, thepresent invention is directed to a novel two layer catalyst compositioncomprising a support material and a precious metal component coated on acarrier support.

BACKGROUND OF THE INVENTION

Operation of lean burn engines, e.g., diesel engines and lean burngasoline engines, provide the user with excellent fuel economy, and havevery low emissions of gas phase hydrocarbons and carbon monoxide due totheir operation at high air/fuel ratios under fuel lean conditions.Diesel engines, in particular, also offer significant advantages overgasoline engines in terms of their fuel economy, durability, and theirability to generate high torque at low speed.

From the standpoint of emissions, however, diesel engines presentproblems more severe than their spark-ignition counterparts. Emissionproblems relate to particulate matter (PM), nitrogen oxides (NOx),unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term usedto describe various chemical species of nitrogen oxides, includingnitrogen monoxide (NO) and nitrogen dioxide (NO₂), among others. NO isof concern because it is believed to undergo a process known asphoto-chemical smog formation, through a series of reactions in thepresence of sunlight and hydrocarbons, and is significant contributor toacid rain. NO₂ on the other hand has a high potential as an oxidant andis a strong lung irritant. Particulates (PM) are also connected torespiratory problems. As engine operation modifications are made toreduce particulates and unburned hydrocarbons on diesel engines, the NOxemissions tend to increase.

The two major components of particulate matter are the volatile organicfraction (VOF) and a soot fraction (soot). The VOF condenses on the sootin layers, and is derived from the diesel fuel and oil. The VOF canexist in diesel exhaust either as a vapor or as an aerosol (finedroplets of liquid condensate) depending on the temperature of theexhaust gas. Soot is predominately composed of particles of carbon. Theparticulate matter from diesel exhaust is highly respirable due to itsfine particle size, which poses health risks at higher exposure levels.Moreover, the VOF contains polycyclic aromatic hydrocarbons, some ofwhich are suspected carcinogens.

A filter known in the art for trapping particulate matter is a wall-flowfilter. Such wall-flow filters can comprise catalysts on the filter andburn off filtered particulate matter. A common construction is amulti-channel honeycomb structure having the ends of alternate channelson the upstream and downstream sides of the honeycomb structure plugged.This results in a checkerboard type pattern on either end. Channelsplugged on the upstream or inlet end are opened on the downstream oroutlet end. This permits the gas to enter the open upstream channels,flow through the porous walls and exit through the channels having opendownstream ends. The gas to be treated passes into the catalyticstructure through the open upstream end of a channel and is preventedfrom exiting by the plugged downstream end of the same channel. The gaspressure forces the gas through the porous structural walls intochannels closed at the upstream end and opened at the downstream end.Such structures are primarily known to filter particles out of theexhaust gas stream. Often the structures have catalysts on thesubstrate, which enhance the oxidation of the particles. Typical patentsdisclosing such catalytic structures include U.S. Pat. Nos. 3,904,551;4,329,162; 4,340,403; 4,364,760; 4,403,008; 4,519,820; 4,559,193; and4,563,414.

One NOx removal technique comprises a non-thermal plasma gas treatmentof NO to produce NO₂ which is then combined with catalytic storagereduction treatment, e.g., a lean NOx trap, to enhance NOx reduction inoxygen-rich vehicle engine exhausts. In the lean NOx trap, the NO₂ fromthe plasma treatment is adsorbed on a nitrate-forming material, such asan alkali or alkaline earth metals, and stored as a nitrate. An enginecontroller unit (ECU) periodically runs a brief fuel-rich condition toprovide hydrocarbons for a reaction that decomposes the stored nitrateinto benign N₂. By using a plasma, the lean NOx trap catalyst can beimplemented with known NOx absorbers, and the catalyst may contain lessor essentially no precious metals, such as Pt, Pd and Rh, for reductionof the nitrate to N₂. Accordingly, an advantage is that a method for NOxemission reduction is provided that is inexpensive and reliable. Theplasma-assisted lean NOx trap can allow the life of precious metal leanNOx trap catalysts to be extended for relatively inexpensive complianceto NOx emission reduction laws. Furthermore, not only does theplasma-assisted lean NOx trap process improve the activity, durability,and temperature window of lean NOx trap catalysts, but it allows thecombustion of fuels containing relatively high sulfur contents with aconcomitant reduction of NOx, particularly in an oxygen-rich vehicularenvironment.

Another catalytic technology for removal of NOx from lean-burn engineexhausts involves NOx storage reduction catalysis, commonly called the“lean-NOx trap”. The lean-NOx trap technology can involve the catalyticoxidation of NO to NO₂ by catalytic metal components effective for suchoxidation, such as precious metals. However, in the lean NOx trap, theformation of NO₂ is followed by the formation of a nitrate when the NO₂is adsorbed onto the catalyst surface. The NO₂ is thus “trapped”, i.e.,stored, on the catalyst surface in the nitrate form and subsequentlydecomposed by periodically operating the system under fuel-richcombustion conditions that effect a reduction of the released NOx(nitrate) to N₂.

Oxidation catalysts comprising a precious metal dispersed on arefractory metal oxide support are known for use in treating the exhaustof diesel engines in order to convert both hydrocarbon and carbonmonoxide gaseous pollutants by catalyzing the oxidation of thesepollutants to carbon dioxide and water. Such catalysts have beengenerally contained in units called diesel oxidation catalysts (DOC), ormore simply catalytic converters, which are placed in the exhaust flowpath from a Diesel-powered engine to treat the exhaust before it ventsto the atmosphere. Typically, the diesel oxidation catalysts are formedon ceramic or metallic substrate carriers (such as the flow-throughmonolith carrier, as described hereinbelow) upon which one or morecatalyst coating compositions are deposited. In addition to theconversions of gaseous HC, CO and the SOF fraction of particulatematter, oxidation catalysts that contain platinum group metals (whichare typically dispersed on a refractory oxide support) promote theoxidation of nitric oxide (NO) to NO₂.

U.S. Pat. No. 5,491,120 discloses oxidation catalysts containing ceriaand a bulk second metal oxide which may be one or more of titania,zirconia, ceria-zirconia, silica, alumina-silica and alpha-alumina.

U.S. Pat. No. 5,627,124 discloses oxidation catalysts containing ceriaand alumina. It is disclosed that each have a surface area of at leastabout 10 m²/g. The weight ratio of ceria to alumina is disclosed to be1.5:1 to 1:1.5. It is further disclosed to optionally include platinum.The alumina is disclosed to preferably be activated alumina. U.S. Pat.No. 5,491,120 discloses oxidation catalysts containing ceria and a bulksecond metal oxide, which may be one or more of titania, zirconia,ceria-zirconia, silica, alumina-silica and alpha-alumina.

The prior art also shows an awareness of the use of zeolites, includingmetal-doped zeolites, to treat diesel exhaust. For example, U.S. Pat.No. 4,929,581 discloses a filter for diesel exhaust, in which theexhaust is constrained to flow through the catalyst walls to filter thesoot particles. A catalyst comprising a platinum group metal-dopedzeolite is dispersed on the walls of the filter to catalyze oxidation ofthe soot to unplug the filter.

As is well-known in the art, catalysts used to treat the exhaust ofinternal combustion engines are less effective during periods ofrelatively low temperature operation, such as the initial cold-startperiod of engine operation, because the engine exhaust is not at atemperature sufficiently high for efficient catalytic conversion ofnoxious components in the exhaust. To this end, it is known in the artto include an adsorbent material, which may be a zeolite, as part of acatalytic treatment system in order to adsorb gaseous pollutants,usually hydrocarbons, and retain them during the initial cold-startperiod. As the exhaust gas temperature increases, the adsorbedhydrocarbons are driven from the adsorbent and subjected to catalytictreatment at the higher temperature. In this regard, see for exampleU.S. Pat. No. 5,125,231 which discloses (columns 5-6) the use ofplatinum group metal-doped zeolites as low temperature hydrocarbonadsorbents as well as oxidation catalysts.

As discussed hereinabove, oxidation catalysts comprising a platinumgroup metal dispersed on a refractory metal oxide support are known foruse in treating exhaust gas emissions from diesel engines. Platinum (Pt)remains the most effective platinum group metal for oxidizing CO and HCin a DOC, after high temperature aging under lean conditions and in thepresence of fuel sulfur. Nevertheless, one of the major advantages ofusing Pd based catalysts is the lower cost of Pd compared to Pt.However, Pd based DOCs typically show higher light-off temperatures foroxidation of CO and HC, especially when used with HC storage materials,potentially causing a delay in HC and or CO light-off. Pd containingDOCs may poison the activity of Pt to convert paraffins and/or oxidizeNO and may also make the catalyst more susceptible to sulfur poisoning.These characteristics have typically prevented the use of Pd as anoxidation catalyst in lean burn operations especially for light dutydiesel applications where engine temperatures remain below 250° C. formost driving conditions.

The present invention conceives of a novel washcoat design in order toimprove the before mentioned disadvantages.

SUMMARY OF THE INVENTION

The present invention is directed to a diesel oxidation catalyst for thetreatment of exhaust gas emissions, such as the oxidation of unburnedhydrocarbons (HC), and carbon monoxide (CO) and the reduction ofnitrogen oxides (NOx). More particularly, the present invention isdirected to a novel washcoat composition comprising two distinctwashcoat layers containing two distinctly different weight ratios ofPt:Pd.

In another embodiment, the present invention is directed to a method oftreating an exhaust gas stream by contacting the exhaust gas stream witha diesel oxidation catalyst containing a novel washcoat compositioncomprising two distinct washcoat layers containing two distinctlydifferent ratios of Pt:Pd.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a honeycomb-type refractory carriermember which may comprise a novel diesel oxidation catalyst (DOC)washcoat composition in accordance with the present invention;

FIG. 2 is a partial cross-sectional view enlarged relative to FIG. 1 andtaken along a plane parallel to the end faces of the carrier of FIG. 1,which shows an enlarged view of one of the gas flow passages shown inFIG. 1;

FIG. 3 is a schematic view showing an alternative configuration ofdiesel oxidation catalyst (DOC) washcoat composition, in accordance withone embodiment of the present invention;

FIG. 4 is a schematic of an engine emission treatment system, inaccordance with one embodiment of the present invention;

FIG. 5 shows a comparison of CO light-off after 5 hour aging at 800° C.between various washcoat compositions;

FIG. 6 shows a comparison of HC light-off after 5 hour aging at 800° C.between various washcoat compositions;

FIG. 7 shows a comparison of CO light-off after 5 hour aging at 800° C.between various washcoat compositions;

FIG. 8 shows a comparison of HC light-off after 5 hour aging at 800° C.between various washcoat compositions;

FIG. 9 shows a comparison of CO light-off after 5 hour aging at 900° C.between various washcoat compositions;

FIG. 10 shows a comparison of HC light-off after 5 hour aging at 900° C.between various washcoat compositions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a novel diesel oxidation catalyst(DOC) washcoat composition comprising two distinct washcoat layers. Thewashcoat composition of the present invention is optimized for sulfurtolerance and paraffin oxidation in the first or top washcoat layer andfor hydrothermal stability in the second or bottom washcoat layer. Thefirst or top washcoat layer comprises a high-surface area supportmaterial, one or more hydrocarbon storage components, and a preciousmetal catalyst containing platinum (Pt) and palladium (Pd). The secondor bottom washcoat layer comprises a high-surface area support materialand a precious metal catalyst containing platinum (Pt) and palladium(Pd), wherein the support is a substantially silica free supportmaterial and does not contain a hydrocarbon storage component.

The present invention is also directed to a method for treating dieselengine exhaust gas stream emissions containing unburned hydrocarbons(HC) and carbon monoxides (CO). An exhaust gas stream from a dieselengine can be treated in an emission treatment device containing thenovel washcoat composition of the present invention. In accordance withthe present invention, the exhaust gas stream first comes into contactwith the first or top washcoat layer and subsequently comes into contactwith the second or bottom washcoat layer.

In accordance with the present invention, two layers with two distinctlydifferent ratios of Pt:Pd are employed wherein the Pt:Pd weight ratio ina first layer (the first or top washcoat layer) is greater than thePt:Pd weight ratio of a second layer (the second or bottom washcoatlayer) (hereinafter “Pt:Pd ratio” refers to a Pt:Pd weight ratio). Forexample, the first or top washcoat layer may contain a Pt:Pd weightratio of at least 2:1. Pt:Pd ratios from at least about 2:1 to about10:1, from about 3:1 to about 5:1, or from about 3:1 to about 4:1, arealso exemplified. It is important to use a high amount of Pt in thefirst or top washcoat layer in order to boost sulfur tolerance whilemaintaining some stabilization of the metal phase against sintering. Thefirst or top washcoat layer contains a hydrocarbon (HC) storagecomponent, e.g., a zeolite, in order to store HCs during the cold startperiod of the drive cycle. After warm-up of the catalyst, thehydrocarbon (HC) storage component will release the stored HCs which aresubsequently converted over the catalyst. It is important that thehydrocarbon (HC) storage component (e.g., zeolite) be incorporated intothe layer with the higher Pt:Pd ratio in order to ensure an efficientconversion of released paraffins. The preparation of the first or topwashcoat layer avoids acetate, in the form of precursors or free acid,in order to achieve better fixation of Pt and Pd in the top washcoat andto minimize the resolubilization of Pd in the bottom washcoat duringcoating, and thus, potential migration of Pd from the bottom washcoatinto the top washcoat during the drying process. Soluble Pd and Pt whichare mobile in the aqueous phase of the slurry might migrate during thedrying process and lead to an accumulation of precious metals at thesurface of the washcoat and at the front end and rear end of thecatalyst. Instead of acetic acid, a small amount of tartaric acid may beused in the top washcoat to fix the precious metals (e.g., Pt and/or Pd)on the support. The use of tartaric acid helps in the achievement ofseparate washcoat layers with different Pt:Pd ratios.

The second or bottom layer contains a lower Pt:Pd ratio to replace amaximum of the Pt with Pd for maximum cost saving reasons. The second orbottom washcoat layer has a Pt:Pd ratio of less than about 2:1. Also,exemplified are Pt:Pd ratios of from less than about 2:1 to about 1:2,or from less than about 2:1 to about 1.4:1 (7:5). However, a minimumratio of 1.4:1 (7:5) is preferred in order to guarantee sufficientCO/olefin light-off activity after thermal aging. The support used inthe second or bottom washcoat layer is a silica-free support (e.g.,alumina or blend of aluminas) in order to prevent silica interferencewith Pd. The bottom layer does not contain any HC storage materials(e.g., a zeolite) in order to prevent the storage and release ofparaffins during a light-off protocol. Released paraffins cannot beconverted efficiently when the Pd content in the catalyst is greaterthan 30 wt % of the total precious metal weight. In one embodiment,lanthanum oxide (e.g., La₂O₃) as a promoter may be used in the second orbottom coat to boost high temperature stability. The use of smallamounts of zirconium oxide (e.g., ZrO₂) in order to stabilize theprecious metal against sintering may be used in both the bottom and topwashcoat layers.

Preferably, the novel oxidation catalyst washcoat composition of thepresent invention is disposed on a substrate. The substrate may be anyof those materials typically used for preparing catalysts, and willpreferably comprise a ceramic or metal honeycomb structure. Any suitablesubstrate may be employed, such as a monolithic substrate of the typehaving fine, parallel gas flow passages extending therethrough from aninlet or an outlet face of the substrate, such that passages are open tofluid flow therethrough (referred to herein as flow-through substrates).The passages, which are essentially straight paths from their fluidinlet to their fluid outlet, are defined by walls on which the catalyticmaterial is coated as a washcoat so that the gases flowing through thepassages contact the catalytic material. The flow passages of themonolithic substrate are thin-walled channels, which can be of anysuitable cross-sectional shape and size such as trapezoidal,rectangular, square, sinusoidal, hexagonal, oval, circular, etc.

Such monolithic carriers may contain up to about 700 or more flowpassages (or “cells”) per square inch of cross section, although farfewer may be used. For example, the carrier may have from about 7 to600, more usually from about 100 to 400, cells per square inch (“cpsi”).The cells can have cross sections that are rectangular, square,circular, oval, triangular, hexagonal, or are of other polygonal shapes.Flow-through substrates typically have a wall thickness between 0.002and 0.1 inches. Preferred flow-through substrates have a wall thicknessof between 0.002 and 0.015 inches.

The ceramic substrate may be made of any suitable refractory material,e.g., cordierite, cordierite-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, alumina, an aluminosilicate andthe like.

The substrates useful for the catalysts of the present invention mayalso be metallic in nature and be composed of one or more metals ormetal alloys. The metallic substrates may be employed in various shapessuch as corrugated sheet or monolithic form. Preferred metallic supportsinclude the heat resistant metals and metal alloys such as titanium andstainless steel as well as other alloys in which iron is a substantialor major component. Such alloys may contain one or more of nickel,chromium and/or aluminum, and the total amount of these metals mayadvantageously comprise at least 15 wt % of the alloy, e.g., 10-25 wt %of chromium, 3-8 wt % of aluminum and up to 20 wt % of nickel. Thealloys may also contain small or trace amounts of one or more othermetals such as manganese, copper, vanadium, titanium and the like. Thesurface or the metal substrates may be oxidized at high temperatures,e.g., 1000° C. and higher, to improve the resistance to corrosion of thealloys by forming an oxide layer on the surfaces the substrates. Suchhigh temperature-induced oxidation may enhance the adherence of therefractory metal oxide support and catalytically promoting metalcomponents to the substrate.

The oxidation catalyst washcoat compositions of the present inventioncan be applied to the substrate surfaces by any known means in the art.For example, the catalyst washcoat can be applied by spray coating,powder coating, or brushing or dipping a surface into the catalystcomposition.

Useful high-surface area supports include one or more refractory oxides.These oxides include, for example, silica and alumina, titania andzirconia include mixed oxide forms such as silica-alumina,aluminosilicates which may be amorphous or crystalline,alumina-zirconia, alumina-ceria and the like and titanium-alumina andzirconium-silicate. In one embodiment, the support is preferablycomprised of alumina which preferably includes the members of the gamma,delta, theta or transitional aluminas, such as gamma and eta aluminas,and, if present, a minor amount of other refractory oxide, e.g., aboutup to 20 weight percent. Desirably, the active alumina has a specificsurface area of 60 to 350 m²/g, and typically 90 to 250 m²/g. Theloading on the refractory oxide support is preferably from about 0.5 toabout 6 g/in³, more preferably from about 2 to about 5 g/in³ and mostpreferably from about 3 to about 4 g/in³.

The high-surface area support material of the first or top washcoatlayer is preferably a refractory oxide material which is selected fromthe group including compounds of silica, alumina, zirconia, titania andmixtures thereof. Particularly preferred supports are activated,high-surface area compounds selected from the group consisting ofalumina, silica, titania, zirconia, silica-alumina, alumina-zirconia,alumina-chromia, alumina-ceria zirconium-silicate and titania-alumina.

The support used in the second or bottom washcoat layer is asubstantially silica-free high-surface area support (e.g., alumina orblend of aluminas) in order to prevent silica poisoning of the Pd. Asused herein, a “substantially silica-free high-surface area support” isa support material containing no more than 10 wt % silica or iscompletely free of silica. In one embodiment, the silica freehigh-surface area support is selected from the group including compoundsof alumina, zirconia, titania and mixtures thereof.

As previously discussed, the catalyst composition of the presentinvention comprises two layers in which two distinctly different ratiosof platinum (Pt) to palladium (Pd) (Pt:Pd) are employed. The first ortop washcoat layer contains a Pt:Pd weight ratio of at least about 2:1and the second or bottom washcoat layer contains a Pt:Pd weight ratio ofless than about 2:1. The total precious metal component loading based ongrams of precious metal per volume of monolith is from 5 to 500 g/ft³,preferably 15 to 250 g/ft³, preferably from 10 to 150 g/ft³.

Optionally, the first or top washcoat layer of the present invention maycontain one or more hydrocarbon (HC) storage component for theadsorption of hydrocarbons (HC). Typically, any known hydrocarbonstorage material can be used, e.g., a micro-porous material such as azeolite or zeolite-like material. Preferably, the hydrocarbon storagematerial is a zeolite. The zeolite can be a natural or synthetic zeolitesuch as faujasite, chabazite, clinoptilolite, mordenite, silicalite,zeolite X, zeolite Y, ultrastable zeolite Y, ZSM-5 zeolite, offretite,or a beta zeolite. Preferred zeolite adsorbent materials have a highsilica to alumina ratio. The zeolites may have a silica/alumina molarratio of from at least about 25/1, preferably at least about 50/1, withuseful ranges of from about 25/1 to 1000/1, 50/1 to 500/1 as well asabout 25/1 to 300/1, from about 100/1 to 250/1, or alternatively fromabout 35/1 to 180/1 is also exemplified. Preferred zeolites include ZSM,Y and beta zeolites. A particularly preferred adsorbent may comprises abeta zeolite of the type disclosed in U.S. Pat. No. 6,171,556incorporated herein by reference in its entirety. The zeolite loadingshould not be smaller than 0.3 g/in³ in order to guarantee sufficient HCstorage capacity and to prevent a premature release of stored paraffinsduring the temperature ramp following a low temperature storage.Preferably, zeolite content is in the range of 0.4-0.7 g/in³. Higherzeolite loadings than 1.0 g/in³ may lead to premature release of storedtoluene if present in the feed. A premature release of aromatics andparaffins from the zeolite may cause a delay in the CO and HC light-off.

In one embodiment, the one or more zeolites may be stabilized by ionexchange with a rare earth metal. In another embodiment, the washcoatlayer(s) of the present invention may include one or more rare earthoxides (e.g., ceria) to promote the oxidation of heavy HCs.

In one embodiment, the washcoat composition of the present inventioncomprises two distinct washcoat layers coated on a single substrate orcarrier member, one layer (e.g., the first or top washcoat layer) overtop of the other (e.g., the second or bottom washcoat layer). In thisembodiment, the second or bottom washcoat layer is coated over theentire axial length of a substrate (e.g., a flow-through monolith) andthe first or top washcoat layer is coated over the entire axial lengthof the second or bottom washcoat layer. In accordance with the presentinvention, the first or top washcoat layer comprises a high-surface areasupport material, one or more zeolites, and a precious metal catalystcontaining platinum (Pt) and palladium (Pd) in a Pt:Pd ratio of at leastabout 2:1. The second or bottom layer comprises a high-surface areasupport material and a precious metal catalyst containing platinum (Pt)and palladium (Pd) in a Pt:Pd ratio of less than about 2:1, wherein saidsupport is a silica-free support material, and wherein said firstwashcoat layer does not contain a hydrocarbon storage component. Thecomplete absence of silica in the support prevents silica inhibition ofPd. Released paraffins are not converted efficiently when the Pt:Pdratio is below 2:1, and as such the second or bottom washcoat does notcontain a HC storage component, which prevents the storage and releaseof paraffins during a light-off period.

The washcoat composition of this embodiment may be more readilyappreciated by reference to FIGS. 1 and 2. FIGS. 1 and 2 show arefractory carrier member 2, in accordance with one embodiment ofpresent invention. Referring to FIG. 1, the refractory carrier member 2is a cylindrical shape having a cylindrical outer surface 4, an upstreamend face 6 and a downstream end face 8, which is identical to end face6. Carrier member 2 has a plurality of fine, parallel gas flow passages10 formed therein. As seen in FIG. 2 flow passages 10 are formed bywalls 12 and extend through carrier 2 from upstream end face 6 todownstream end face 8, the passages 10 being unobstructed so as topermit the flow of a fluid, e.g., a gas stream, longitudinally throughcarrier 2 via gas flow passages 10 thereof. As more easily seen in FIG.2 walls 12 are so dimensioned and configured that gas flow passages 10have a substantially regular polygonal shape, substantially square inthe illustrated embodiment, but with rounded corners in accordance withU.S. Pat. No. 4,335,023, issued Jun. 15, 1982 to J. C. Dettling et al. Adiscrete bottom layer 14, which in the art and sometimes below isreferred to as a “washcoat”, is adhered or coated onto the walls 12 ofthe carrier member. As shown in FIG. 2, a second discrete top washcoatlayer 16 is coated over the bottom washcoat layer 14. In accordance withthe present invention, the top washcoat layer 16 comprises ahigh-surface area support material, one or more zeolites, and a preciousmetal catalyst containing platinum (Pt) and palladium (Pd) in a Pt:Pdratio of at least about 2:1. The bottom washcoat layer 16 comprises ahigh-surface area support material and a precious metal catalystcontaining platinum (Pt) and palladium (Pd) in a Pt:Pd ratio of lessthan about 2:1, wherein said support is a silica free support material,and wherein said first washcoat layer does not contain a hydrocarbonstorage component.

As shown in FIG. 2, the carrier member include void spaces provided bythe gas-flow passages 10, and the cross-sectional area of these passages10 and the thickness of the walls 12 defining the passages will varyfrom one type of carrier member to another. Similarly, the weight ofwashcoat applied to such carriers will vary from case to case.Consequently, in describing the quantity of washcoat or catalytic metalcomponent or other component of the composition, it is convenient to useunits of weight of component per unit volume of catalyst carrier.Therefore, the units grams per cubic inch (“g/in³”) and grams per cubicfoot (“g/ft³”) are used herein to mean the weight of a component pervolume of the carrier member, including the volume of void spaces of thecarrier member.

During operation, exhaust gaseous emissions from a lean burn enginecomprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfuroxides initially encounter the top washcoat layer 16, and thereafterencounter the bottom washcoat layer 14.

In another embodiment, the distinct washcoat layers of the presentinvention may be zone coated such that one washcoat layer is on theupstream end, and the other washcoat on the downstream end, of thecarrier substrate. For example, an upstream washcoat layer can be coatedover a portion of the upstream region of the substrate and a downstreamwashcoat layer can be coated over a downstream portion of the substrate.In this embodiment, the top washcoat layer of the present invention iscoated over the upstream potion of the carrier substrate (i.e., theupstream washcoat layer) and the bottom washcoat layer is coated over adownstream portion of the carrier substrate (i.e., the downstreamwashcoat layer). The upstream washcoat layer comprises a high-surfacearea support material, one or more zeolites, and a precious metalcatalyst containing platinum (Pt) and palladium (Pd) in a Pt:Pd ratio ofat least about 2:1. The second or bottom washcoat layer comprises ahigh-surface area support material and a precious metal catalystcontaining platinum (Pt) and palladium (Pd) in a Pt:Pd ratio of lessthan about 2:1. In accordance with the present invention it is essentialthat the downstream washcoat layer does not contain a silica supportmaterial, and does not contain a hydrocarbon storage component. Thecomplete absence of silica in the support prevents silica poisoning ofPd. Released paraffins are not converted efficiently when the Pd contentis higher than 30 wt %, and as such the second or bottom washcoat doesnot contain a HC storage component, which prevents the storage andrelease of paraffins during a light-off period.

The catalyst composition of this embodiment may be more easilyunderstood by reference to FIG. 3. As shown in FIG. 3 a diesel oxidationcatalyst 20 is coated with a novel washcoat composition comprising acarrier member or substrate 22, for example a honeycomb monolith, whichcontains two separate zone coated washcoated layers, an upstreamwashcoat layer 24 and a downstream washcoat layer 26. The upstream layer24 contains a high-surface area support material, one or more zeolites,and a precious metal catalyst containing platinum (Pt) and palladium(Pd) in a Pt:Pd ratio of at least about 2:1. The downstream washcoatlayer 26 may contain a high-surface area support material and a preciousmetal catalyst containing platinum (Pt) and palladium (Pd) in a Pt:Pdratio of less than about 2:1. In accordance with the present inventionit is essential that the downstream washcoat layer does not contain asilica support material, and does not contain a hydrocarbon storagecomponent. Both the upstream washcoat layer 24 and downstream washcoatlayer 26, respectively, generally contain a precious metal loading offrom about 5 to 500 g/ft³. Loadings of precious metal from 25 to 250g/ft³ and 60 to 150 g/ft³ are also exemplified.

In this embodiment, the upstream 24 and downstream 26 washcoat layers,respectively, are each zone coated only over a portion of the substrate23. A typical substrate being from about 3 to about 10 inches in length,substrates having a length of about 5 inches or about 7 inches are alsoexemplified. However, the combination of the upstream 24 and downstream26 washcoat layers, respectively, cover the entire length of thesubstrate 23. The upstream washcoat layer 24 can be coated over at least0.5 inch, and up to 5 inches, of the upstream portion of the substrate23. An upstream washcoat layer 24 having a length of at least about 1.0inch, and up to 3.5 inches, or from at least 1.5 inches and up to 2.5inches, from the upstream edge of the catalytic member, are alsoexemplified. With the downstream washcoat portion 26 covering theremaining downstream portion of the substrate 23.

The length of the upstream washcoat layer 24 can also be described as apercentage of the length of the catalytic member from the upstream todownstream edge. Typically, the upstream washcoat layer 24 will comprisefrom about 5 to about 70% of the upstream length of the catalyticmember. Also exemplified is an upstream washcoat layer 24 of up to about20%, up to about 40%, and up to about 60% of the upstream length of thediesel oxidation catalyst 20. With the downstream washcoat portion 26covering the remaining downstream portion of the substrate 22. Thus, thedownstream washcoat portion 26 may comprise 95 to about 30% of thedownstream portion 30 of the substrate 23.

During operation, exhaust gases flow through the diesel oxidationcatalyst 20 from the upstream edge 25 to the down stream edge 27. Theprecious metal catalysts contained in both the upstream 24 anddownstream 26 washcoat layers, respectively, oxidize HC and COpollutants contained in the exhaust gases.

The diesel oxidation catalyst (DOC) of the present invention can be usedin an integrated emission treatment system comprising one or moreadditional components for the treatment of diesel exhaust gas emissions.For example, the emission treatment system may further comprise a sootfilter component and/or a selective catalytic reduction (SCR) component.The diesel oxidation catalyst can be located upstream or downstream fromthe soot filter and/or selective catalytic reduction component.

In addition to treating the exhaust gas emissions via use of anoxidation catalyst the present invention may employ a soot filter forremoval of particulate matter. The soot filter may be located upstreamor downstream from the DOC, but is preferably located downstream fromthe diesel oxidation catalyst. In a preferred embodiment, the sootfilter is a catalyzed soot filter (CSF). The CSF of the presentinvention comprises a substrate coated with a washcoat layer containingone or more catalysts for burning off trapped soot and or oxidizingexhaust gas stream emissions. In general, the soot burning catalyst canbe any known catalyst for combustion of soot. For example, the CSF canbe coated with a one or more high surface area refractory oxides (e.g.,alumina, silica, silica alumina, zirconia, and zirconia alumina) and/oran oxidation catalyst (e.g., a ceria-zirconia) for the combustion ofunburned hydrocarbons and to some degree particulate matter. However,preferably the soot burning catalyst is an oxidation catalyst comprisingone or more precious metal (PM) catalysts (platinum, palladium, and/orrhodium).

In general, any known filter substrate in the art can be used,including, e.g., a honeycomb wall flow filter, wound or packed fiberfilter, open-cell foam, sintered metal filter, etc., with wall flowfilters being preferred. Wall flow substrates useful for supporting theCSF compositions have a plurality of fine, substantially parallel gasflow passages extending along the longitudinal axis of the substrate.Typically, each passage is blocked at one end of the substrate body,with alternate passages blocked at opposite end-faces. Such monolithiccarriers may contain up to about 700 or more flow passages (or “cells”)per square inch of cross section, although far fewer may be used. Forexample, the carrier may have from about 7 to 600, more usually fromabout 100 to 400, cells per square inch (“cpsi”). The cells can havecross sections that are rectangular, square, circular, oval, triangular,hexagonal, or are of other polygonal shapes. Wall flow substratestypically have a wall thickness between 0.002 and 0.1 inches. Preferredwall flow substrates have a wall thickness of between 0.002 and 0.015inches.

Preferred wall flow filter substrates are composed of ceramic-likematerials such as cordierite, α-alumina, silicon carbide, siliconnitride, zirconia, mullite, spodumene, alumina-silica-magnesia orzirconium silicate, or of porous, refractory metal. Wall flow substratesmay also be formed of ceramic fiber composite materials. Preferred wallflow substrates are formed from cordierite, silicon carbide and aluminumtitanate. Such materials are able to withstand the environment,particularly high temperatures, encountered in treating the exhauststreams.

Preferred wall flow substrates for use in the inventive system includethin porous walled honeycombs (monolith)s through which the fluid streampasses without causing too great an increase in back pressure orpressure across the article. Normally, the presence of a clean wall flowarticle will create a back pressure of I inch water column to 10 psig.Ceramic wall flow substrates used in the system are preferably formed ofa material having a porosity of at least 40% (e.g., from 40 to 70%)having a mean pore size of at least 5 microns (e.g., from 5 to 30microns). More preferably, the substrates have a porosity of at least50% and have a mean pore size of at least 10 microns. When substrateswith these porosities and these mean pore sizes are coated with thetechniques described below, adequate levels of the CSF catalystcompositions can be loaded onto the substrates to achieve excellent NOxconversion efficiency and burning off of soot. These substrates arestill able to retain adequate exhaust flow characteristics, i.e.,acceptable back pressures, despite the CSF catalyst loading. U.S. Pat.No. 4,329,162 is herein incorporated by reference with respect to thedisclosure of suitable wall flow substrates.

The porous wall flow filter used in this invention is optionallycatalyzed in that the wall of said element has thereon or containedtherein one or more catalytic materials, such CSF catalyst compositionsare described hereinabove. Catalytic materials may be present on theinlet side of the element wall alone, the outlet side alone, both theinlet and outlet sides, or the wall itself may consist all, or in part,of the catalytic material. In another embodiment, this invention mayinclude the use of one or more washcoat layers of catalytic materialsand combinations of one or more washcoat layers of catalytic materialson the inlet and/or outlet walls of the element.

The exhaust gas treatment system of the present invention may furthercomprise a selective catalytic reduction (SCR) component. The SCRcomponent may be located upstream or downstream of the DOC and/or sootfilter. Preferably, the SCR component is located downstream of a sootfilter component. A suitable SCR catalyst component for use in theemission treatment system is able to effectively catalyze the reductionof the NOx component at temperatures below 600° C., so that adequate NOxlevels can be treated even under conditions of low load which typicallyare associated with lower exhaust temperatures. Preferably, the catalystarticle is capable of converting at least 50% of the NOx component toN₂, depending on the amount of reductant added to the system. Anotherdesirable attribute for the composition is that it possesses the abilityto catalyze the reaction of O₂ with any excess NH₃ to N₂ and H₂O, sothat NH₃ is not emitted to the atmosphere. Useful SCR catalystcompositions used in the emission treatment system should also havethermal resistance to temperatures greater than 650° C. Such hightemperatures may be encountered during regeneration of the upstreamcatalyzed soot filter.

Suitable SCR catalyst compositions are described, for instance, in U.S.Pat. No. 4,961,917 (the '917 patent) and U.S. Pat. No. 5,516,497, whichare both hereby incorporated by reference in their entirety.Compositions disclosed in the '917 patent include one or both of an ironand a copper promoter present in a zeolite in an amount of from about0.1 to 30 percent by weight, preferably from about 1 to 5 percent byweight, of the total weight of promoter plus zeolite. In addition totheir ability to catalyze the reduction of NOx with NH₃ to N₂, thedisclosed compositions can also promote the oxidation of excess NH₃ withO₂, especially for those compositions having higher promoterconcentrations.

In one embodiment, the present invention is directed to an emissiontreatment system comprising one or more additional components for thetreatment of diesel exhaust gas emissions. An exemplified emissiontreatment system may be more readily appreciated by reference to FIG. 4,which depicts a schematic representation of an emission treatment system32, in accordance with this embodiment of the present invention.Referring to FIG. 4, an exhaust gas stream containing gaseous pollutants(e.g., unburned hydrocarbons, carbon monoxide and NOx) and particulatematter is conveyed via line 36 from an engine 34 to a diesel oxidationcatalyst (DOC) 38, which is coated with the novel washcoat compositionof the present invention. In the DOC 38, unburned gaseous andnon-volatile hydrocarbons (i.e., the VOF) and carbon monoxide arelargely combusted to form carbon dioxide and water. In addition, aproportion of the NO of the NOx component may be oxidized to NO₂ in theDOC. The exhaust stream is next conveyed via line 40 to a catalyzed sootfilter (CSF) 42, which traps particulate matter present within theexhaust gas stream. The CSF 42 is optionally catalyzed for passiveregeneration. After removal of particulate matter, via CSF 42, theexhaust gas stream is conveyed via line 44 to a downstream selectivecatalytic reduction (SCR) component 16 for the treatment and/orconversion of NOx.

EXAMPLES Example 1 Washcoat Composition A

A top washcoat, containing 1.5 g/in³ of 1.5 wt % silica alumina having asurface area of 100 m²/g as a precious metal support and Pt and Pd in a2:1 ratio (73.3 g/ft³ Pt and 36.67 g/ft³ Pd), was coated onto a carriersubstrate. A small amount of ZrO₂ was implemented as a blend of Zr—OH(0.05 g/in³) and Zr—N (0.005 g/in³). Acetic acid or acetate precursorswere strictly avoided in the top washcoat so as to avoidresolubilization of the Pd and part of the Pt in the bottom and topwashcoats. A small amount of tartaric acid was used in the top washcoatto fix precious group metals on the support.

Example 2 Washcoat Composition B

A top washcoat, containing 1.5 g/in³ silica alumina having a surfacearea of 100 m²/g as a precious metal support and Pt and Pd in a 2:1ratio (73.3 g/ft³ Pt and 36.67 g/ft³ Pd), was coated onto a carriersubstrate. A medium loading (0.4 g/in³) of zeolite H-Beta (spray dried)was used in the top washcoat. A small amount of ZrO₂ was implemented asa blend of Zr—OH (0.05 g/in³) and Zr—N (0.005 g/in³). Acetic acid oracetate precursors were strictly avoided in the top washcoat so as toavoid resolubilization of the Pd and part of the Pt in the bottom andtop washcoats. A small amount of tartaric acid was used in the topwashcoat to fix precious group metals on the support.

Example 3 CO/HC Light-Off Testing

Washcoat compositions A and B were tested for CO light-off and for HCconversion. The gas composition used for determining light-offtemperatures comprised 1500 ppm CO, 100 ppm NO, 14% O₂, 4% CO₂, 4% H₂Owith the balance being N₂. The gas also comprised 120 ppm Cl as propene,80 ppm as toluene, 200 ppm Cl as decane and 30 ppm as methane. The gaswas run over washcoat compositions C-G at a temperature ramp of 20°C./min and a space velocity of 55 k/h. HC conversion as a percentage wascalculated from the gas exiting the catalyst.

CO/HC light-off temperatures were determined for washcoat compositions Aand B after aging at 800° C. for 5 hours (FIGS. 5 and 6). As can be seenfrom FIG. 5 washcoat composition B, which contained no zeolite, had alower CO light-off than washcoat composition A. The HC light-offtemperature was also lowered in washcoat composition B when compared towashcoat composition A (see FIG. 6).

Example 4 Washcoat Composition C (Comparative)

A bottom washcoat, containing 1.5 g/in³ silica alumina having a surfacearea of 100 m²/g, 0.5 g/in³ zeolite H-Beta and 0.1 g/in³ ZrO₂ (asacetate), was coated onto a carrier substrate. The bottom washcoat layerdid not contain any precious metals.

A top washcoat, containing 1.5 g/in³ silica alumina having a surfacearea of 100 m²/g as a precious metal support, 0.5 g/in³ zeolite H-Betaand 110 g/ft³ of Pt, was coated over the bottom washcoat. 0.05 g/in³ ofZrO₂ (as acetate) was used as a binder and to stabilize the PM phaseagainst sintering.

Example 5 Washcoat Composition D (Comparative)

A bottom washcoat, containing 1.5 g/in³ silica alumina having a surfacearea of 100 m²/g, 0.5 g/in³ zeolite H-Beta and 0.1 g/in³ ZrO₂ (asacetate), was coated onto a carrier substrate. The bottom washcoat layerdid not contain any precious metals.

A top washcoat, containing 1.5 g/in³ silica alumina having a surfacearea of 100 m²/g as a precious metal support, 0.5 g/in³ zeolite H-Betaand Pt and Pd in a 2:1 ratio (73.3 g/ft³ Pt and 36.7 g/ft³ Pd), wascoated over the bottom washcoat. 0.05 g/in³ of ZrO₂ (as acetate) wasused as a binder and to stabilize the PM phase against sintering.

Example 6 Washcoat Composition E

A bottom washcoat, containing 0.75 g/in³ of a large pore gamma aluminahaving a surface area of 200 m²/g, 0.75 g/in³ of 70 m²/g delta thetaalumina and Pt and Pd in a 1.4:1 ratio (32.1 g/ft³ of Pt and 23.9 g/ft³Pd), was coated onto a carrier substrate. 0.05 g/in³ of ZrO₂ (asacetate) was used as a binder and to stabilize the PM phase againstsintering. No zeolite or any other siliceous material was used in thebottom coat.

A top washcoat, containing 1.3 g/in³ silica alumina having a surfacearea of 100 m²/g as a precious metal support and Pt and Pd in a 3:1ratio (41.3 g/ft³ Pt and 13.8 g/ft³ Pd), was coated over the bottomwashcoat. A medium loading (0.4 g/in³) of zeolite H-Beta (spray dried)was used in the top washcoat. A small amount of ZrO₂ was implemented asa blend of Zr—OH (0.05 g/in³) and Zr—N (0.005 g/in³). Acetic acid oracetate precursors were strictly avoided in the top washcoat so as toavoid resolubilization of the Pd and part of the Pt in the bottom andtop washcoats. A small amount of tartaric acid was used in the topwashcoat to fix precious group metals on the support.

Example 7 Washcoat Composition F

A bottom washcoat, containing 0.75 g/in³ of a large pore La₂O₃ doppedgamma alumina having a surface area of 200 m²/g, 0.75 g/in³ of 70 m²/gdelta theta alumina and Pt and Pd in a 1.4:1 ratio (32.1 g/ft³ of Pt and23.9 g/ft³ Pd), was coated onto a carrier substrate. The PM supported onthe alumina was coated onto a carrier substrate. 0.05 g/in³ of ZrO₂ (asacetate) was used as a binder and to stabilize the PM phase againstsintering. No zeolite or any other siliceous material was used in thebottom coat.

A top washcoat, containing 1.3 g/in³ silica alumina having a surfacearea of 100 m²/g as a precious metal support and Pt and Pd in a 3:1ratio (41.3 g/ft³ Pt and 13.8 g/ft³ Pd), was coated over the bottomwashcoat. A medium loading (0.4 g/in³) of zeolite H-Beta (spray dried)was used in the top washcoat. A small amount of ZrO₂ was implemented asa blend of Zr—OH (0.05 g/in³) and Zr—N (0.005 g/in³). Acetic acid oracetate precursors were strictly avoided in the top washcoat so as toavoid resolubilization of the Pd and part of the Pt in the bottom andtop washcoats. A small amount of tartaric acid was used in the topwashcoat to fix precious group metals on the support.

Example 8 HC/CO Light-Off Testing

Washcoat compositions C—F were tested for CO light-off and for HCconversion. The gas composition used for determining light-offtemperatures comprised 1500 ppm CO, 100 ppm NO, 14% O₂, 4% CO₂, 4% H₂Owith the balance being N₂. The gas also comprised 120 ppm Cl as propene,80 ppm as toluene, 200 ppm Cl as decane and 30 ppm as methane. The gaswas run over washcoat compositions C-G at a temperature ramp of 20°C./min and a space velocity of 55 k/h. HC conversion as a percentage wascalculated from the gas exciting the catalyst.

HC/CO light-off temperatures were determined for washcoat compositionsC—F after aging at 800° C. for 5 hours (FIGS. 7 and 8) and for washcoatcompositions C, D and F after aging at 900° C. for 5 hours (FIGS. 9 and10). As can be seen from the FIGS. 7 and 9, both washcoat compositions Eand F, which comprised novel washcoat compositions of the presentinvention, had lower CO light-off temperatures than either of thecomparative washcoat compositions C and D. Furthermore, as can be seenfrom FIG. 7 washcoat composition F, which contained La₂O₃ as a promoterto boost HT-stability in the bottom washcoat layer resulted in aslightly lower CO light-off temperature. As can be seen from FIG. 8washcoat composition F had a lower HC light-off temperature thancomparative washcoat compositions C or D. Washcoat composition E alsohad a lower light-off temperature than comparative washcoat compositionsC and D, but the light-off temperature of E was slightly higher thanwashcoat composition F. As can be seen from FIG. 10 washcoat compositionF, after aging at 900° C. for 5 hours, had a HC light-off temperaturelower than comparative Pt/Pd washcoat composition D but higher thancomparative Pt washcoat composition C.

Example 9 Calcination and Aging of Washcoat Composition F

The elemental composition of both the top washcoat (TC) and bottomwashcoat (BC) of washcoat composition F was determined by XPS (X-rayphotoelectron spectroscopy) after calcination and aging for 5 hours at800° C. Both the Pt and Pd precious metals were present in the BC and TCas oxides after calcination (see Table 1). However, after aging at 800°C. for 5 hours the Pt and Pd had been completely reduced to theirmetallic states indicating that the more thermally stable Pt—Pd alloy isformed (see Table 1).

After aging the particle size of the BC was determined by X-raydiffraction to be 16.4 nm and the TC was determined to be 21.2 nmreflecting the higher dispersion and stability with the higher Pd ratio.

TABLE 1 BC and TC elemental composition by XPS after calcination and 5 haging at 800 C BC (cal) BC (aged) TC (cal) BC (aged) Pt⁰ ND 0.03 ND 0.06Pt(OH)₄ 0.08 ND 0.14 ND Pd⁰ ND 0.09 ND 0.14 Pt²⁺ 0.27 ND 0.15 ND LaO_(x)0.4 0.4 ND ND ZrO₂ 1.1 0.9 0.7 0.6

1. A diesel oxidation catalyst for abatement of exhaust gas emissionsfrom a diesel engine comprising: (a) a carrier substrate; (b) a bottomwashcoat layer coated on said carrier substrate comprising ahigh-surface area support material and a precious metal catalystcontaining platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio,wherein said support is a substantially silica-free high-surface areasupport material, and wherein said bottom washcoat layer does notcontain a hydrocarbon storage component; (c) a top washcoat layer coatedover said bottom washcoat layer comprising a high-surface area supportmaterial, one or more hydrocarbon storage components, and a preciousmetal catalyst containing platinum (Pt) and palladium (Pd) in a Pt:Pdweight ratio; and (d) wherein said Pt:Pd weight ratio in said topwashcoat layer is greater than said Pt:Pd weight ratio of said bottomwashcoat layer.
 2. The diesel oxidation catalyst of claim 1, whereinsaid Pt:Pd ratio of said bottom washcoat layer is from less than about2:1 to about 1:2.
 3. The diesel oxidation catalyst of claim 1, whereinsaid Pt:Pd ratio of said top washcoat layer is from at least about 2:1to about 10:1.
 4. The diesel oxidation catalyst of claim 1, wherein saidPt:Pd ratio of said bottom washcoat layer is 7:5 and said Pt:Pd ratio ofsaid top washcoat layer is 3:1.
 5. The diesel oxidation catalyst ofclaim 1, wherein said bottom washcoat and/or top washcoat layer furthercomprises zirconium oxide.
 6. The diesel oxidation catalyst of claim 1,wherein said bottom washcoat layer further comprises lanthanum oxide. 7.The diesel oxidation catalyst of claim 1, wherein said support materialof said bottom washcoat layer is selected from the group consisting ofalumina, zirconia, titania and mixtures thereof.
 8. The diesel oxidationcatalyst of claim 1, wherein said support material of said top washcoatlayer is selected from the group consisting of silica, alumina,zirconia, titania and mixtures thereof.
 9. The diesel oxidation catalystof claim 1, wherein said hydrocarbon storage component is a zeolite andsaid zeolite is selected from the group consisting of faujasite,chabazite, clinoptilolite, mordenite, silicalite, zeolite X, zeolite Y,ultrastable zeolite Y, ZSM zeolite, offretite, and beta zeolite.
 10. Adiesel oxidation catalyst for abatement of exhaust gas emissions from adiesel engine comprising: (a) a carrier substrate; (b) an upstreamwashcoat layer coated over an upstream portion of said carrier substratecomprising a high-surface area support material, one or hydrocarbonstorage components, and a precious metal catalyst containing platinum(Pt) and palladium (Pd) in a Pt:Pd weight ratio; (c) a downstreamwashcoat layer zone coated over a downstream portion of said carriersubstrate comprising a high-surface area support material and a preciousmetal catalyst containing a platinum (Pt) and palladium (Pd) in a Pt:Pdweight ratio, wherein said support is a substantially silica-freehigh-surface area support material, and wherein said downstream washcoatlayer does not contain a hydrocarbon storage component; and (d) whereinsaid Pt:Pd weight ratio in said upstream washcoat layer is greater thansaid Pt:Pd weight ratio of said downstream washcoat layer.
 11. Thediesel oxidation catalyst of claim 10, wherein said Pt:Pd ratio of saiddownstream washcoat layer is from less than about 2:1 to about 1:2. 12.The diesel oxidation catalyst of claim 10, wherein said Pt:Pd ratio ofsaid upstream washcoat layer is from at least about 2:1 to about 10:1.13. The diesel oxidation catalyst of claim 10, wherein said Pt:Pd ratioof said upstream washcoat layer is 3:1 and said Pt:Pd ratio of saiddownstream washcoat layer is 7:5.
 14. The diesel oxidation catalyst ofclaim 1, wherein said hydrocarbon storage component is a zeolite andsaid zeolite is selected from the group consisting of faujasite,chabazite, clinoptilolite, mordenite, silicalite, zeolite X, zeolite Y,ultrastable zeolite Y, ZSM zeolite, offretite, and beta zeolite
 15. Thediesel oxidation catalyst of claim 10, wherein said substrate has anupstream edge and a downstream edge and said top washcoat layer is zonecoated over a length from about 5% to about 70% of said substrate fromsaid upstream edge.
 16. The diesel oxidation catalyst of claim 14,wherein said bottom washcoat layer is zone coated over a length fromabout 95% to about 30% of said substrate from said downstream edge. 17.A method for treating a diesel exhaust gas stream, the method comprisingthe steps of: (a) providing a diesel oxidation catalyst comprising; (i)a carrier substrate having an upstream edge and a downstream edge; (ii)a bottom washcoat layer coated on said carrier substrate comprising ahigh-surface area support material and a precious metal catalystcontaining platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio,wherein said support is a substantially silica-free high-surface areasupport material, and wherein said bottom washcoat layer does notcontain a hydrocarbon storage component; (iii) a top washcoat layercoated over said bottom washcoat layer comprising a high-surface areasupport material, one or hydrocarbon storage components, and a preciousmetal catalyst containing platinum (Pt) and palladium (Pd) in a Pt:Pdweight ratio; and (iv) wherein said Pt:Pd weight ratio in said topwashcoat layer is greater than said Pt:Pd weight ratio of said bottomwashcoat layer; and (b) contacting said diesel exhaust gas stream withsaid diesel oxidation catalyst from said upstream edge to saiddownstream edge for the treatment of exhaust gas emissions.
 18. Themethod of claim 17, wherein said Pt:Pd ratio of said bottom washcoatlayer is 7:5 and said Pt:Pd ratio of said top washcoat layer is 3:1. 19.The method of claim 17, wherein said diesel exhaust gas streamsubsequent to contacting said diesel oxidation catalyst is directed to acatalyzed soot filter (CSF) located downstream of said diesel oxidationcatalyst.
 20. The method of claim 19, wherein diesel exhaust gas streamsubsequent to contacting said catalyzed soot filter (CSF) is directed toa selective catalytic reduction (SCR) component located downstream ofsaid catalyzed soot filter (CSF).